Process of bonding a metal film to a thermoplastic sheet and resulting product



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United States Patent F PROCESS OF BONDING A METAL FILM TO ATHERMOPLASTIC SHEET AND RESULTING PRODUCT David B. Peck and PrestonRobinson, Williamstown, Mass, assignors to Sprague Electric Company,North Adams, Mass, a corporation of Massachusetts NoDrawing. ApplicationJuly 16, 1951, Serial No. 237,050

9 Claims. (Cl. 117-227) This invention relates to new and improvedcondenser sections, and to methods of manufacturing these sections. Thisapplication is a continuation in part of copending applications bearingSerial Numbers 118,198 and 204,546, filed September 27, 1949, andJanuary 5, 1951, respectively.

A great many attempts have been made to manufacture condenser sectionswithout the use of separate electrode foils by applying a metal film toa dielectric sheet. None of these attempts have proved completedsatisfactory because of the limitations of the specific dielectricsused.

Frequently the physical properties of the dielectric material are suchthat the metal film is not adequately bonded to the matrix surface, oris bonded in such a manner that handling of the metallized dielectricwill cause flaws in the metal film. In other cases, the electricalproperties of the dielectric sheet are undesirable in one or moreaspects, for example, the maximum temperature for operation may be lowerthan that desired. Paper, as a supporting dielectric sheet, cannot beoperated at temperatures in excess of 125 C. for extended periods, andeven at 125 C. is subject to early failure. It is necessary toimpregnate metallized paper capacitors, usually with a hydrocarbon wax.

It is an object of this invention to overcome these and other defects ofcondensers employing metallized dielectrics. Another object is toproduce condenser sections which are superior to the metallized filmtype condensers which are presently produced. These and other objects ofthe invention will be apparent from the specification and the annexedclaims.

The aforegoing objects are attained by virtue of the discovery thatmetallic films under certain conditions may be firmly bonded to sheetsof substantially linear polyester polymers conforming to the generalrecurrent formula:

in which R represents a bivalent radical containing the benzene nucleusand in which R represents a bivalent aliphatic radical containing'from 2to 8 carbon atoms. The molecular orientation in these sheets is of primeimportance in obtaining a satisfactory bond. On highly planar orientedsheets of these polyesters the metal deposit sticks in lines roughlycorresponding to the lines of the molecules in the oriented surface. Themetal adhesion is low on this type of surface except along the molecularchains and the metal film resistance is high be- The invention hereinvolved consists of making the molecules on the surfaces of such resinsheets randomly oriented so that a uniform and adequate bond existsbetween the metal film and the resin sheet and that. a uniformelectrical resistance exists in the metal film.

According to one modification of the invention metallizing of apolyester sheet of the indicated variety is carried out undertemperature conditions such that the molecular planar orientation of thesheet is not disturbed 2,740,732 Patented Apr. 3, 1956 A uniform bondbetween the metal film and the matrix surface is not achieved untilafter this heat treatment.

Another embodiment of the invention is concerned with the production ofmetallic film on the indicated type of polyester base by a metalliizngprocess such that the molecular orientation of the resin surface only isrelaxed at the moment of condensation of a metal vapor on this surface.

A third embodiment of the invention of particular utility in formingterminal connections, involved a metal to resin bond obtained byspraying a heavy metal layer directly onto a resinsurface while thelatter is held at a temperature sufficient to completely relax anddisorient the molecular arrangement of this surface.

A preferred dielectric film for use herein is a linear condensationpolymer of ethylene glycol and terephthalic acid. This resin is soldunder the trade name Terylene. Other acceptable polyesters are thecondensation polymers of terephthalic acid with propylene or butyleneglycol, and of chlorinated or otherwise substituted di-basic aromaticacids and substituted glycols, which will condense to form linearpolymers.

With metal bonds of the type mentioned with the initial embodiment ofthe invention, polyester films of the indicated variety are continuouslymetallized in a reduced pressure enclosure, usually with a cooling panelagainst which the back of the film rests at the moment of metallizing.

With most metallic films. it is preferable to first apply a thincondensed undercoat of silver or molybdenum to the resin surface. Thisapplication of metal as well as the subsequent deposition isaccomplished under conditions such that the orientation of the resin,including the surface, is unaffected by the heat of condensation of themetal vapor and the layer provides nuclei for subsequent metaldeposition. Copper, zinc, lead, aluminum, and other metals and incertain special cases, alloys may then be metallized over this base coatin a similar manner, that is, under conditions such'that the heat ofcondensation does not alter the film orientation.

A firm bond between the polyesters and the metal films may be createdafter this step by heating the metallized resin at a temperaturesufiicient to relax'the molecular orientation, at least on the surfaceof the film. This heating may be carried out in an oven or oil bath asdescribed in the copending Robinson and Peck application, Serial Number237,049, filed July 16, 1951 (now Patent No. 2,735,970, granted February21, 195 6), after wound condenser units have been formed. With Terylenefilms treatment temperatures of from 165 C. to 218 C. give satisfactoryresults.

In an example of this embodiment 1 mil Terylene films are first coatedwith a sufficient silver to provide the surface of the resin withcrystal forming nuclei in reduced pressure me'tallizing machine runningunder 1000 microns of mercury pressure. A film speed of feet per minutewas used and the film was backed by a water cooled smooth platen at thepoint of Zinc deposition. Then, a zinc coating is evaporated from a potof the molten metal and condensed on the cool film under the sameconditions, in the same operation. Suflicient metal is used to obtain ashiny deposit by this step. Films which have been coated in this mannerare then heated (as in an oven) to 200 C. for a period of 30 minutes torelax and partially disorient the polyester surface.

This last heating operation need not be carried out in an oven bath oran oil bath. Heated rollers accomplish the same results.

Bonds of the type forming the second preferred embodlment of thisinvention may be formed by following the same procedure indicated in theabove example, ex-

of the bonded product.

araonsa cept that the deposition conditions are modified such that theheat of condensation of the metal vapor is sufficient to relay thesurface of the polyester film without relaxing the inner polyester mass.This may be accomplished, forexample, through film speed decrease and/or the metal vapor temperature increase and, of course, the subsequentheating step may be omitted. By proper operation of this procedure thetemperature of the principal portion of the Terylene film does notadvance beyond 130 C. in temperature, while the surface of the polyesteris approximately 200 C. It is to be understood that these temperaturesare estimates based upon the final analysis With other resins besidesTerylene" which conform to the indicated groups, other treatmentconditions may be employed such that merely the surface of the polyesteris randomly oriented. The same metals and procedures may be used withthis modification as were used in the preceding embodiment.

For application of heavy metal layers, as in the provision of terminalconnections to rolled capacitor edges, these same metals and also alloysmay be sprayed directly onto the edges of the polyester film of theindicated variety under conditions such that the surface of the film isat a temperature above its melting point. Heated vapors and particles ofthese metals can be used in this manner. A firm fused metal to resinbond is obtained because of the interspersing of metal particles in themolten resin mass, which upon cooling, is, of course, disoriented.

As an example of this modification of the invention edges of capacitorswound with 1 mil Terylene film can be provided with terminal connectionsby being sprayed with zinc metal whose particle temperature at themoment of contact with the film is between about 250 C. and about 420 C.at atmospheric pressure. According to a preferred embodiment of theinvention, the metallized dielectric fim is assembled into capacitorform and then heated above the relaxation temperature. This isparticularly desirable when the metal deposition has been accomplishedwithout relaxation of the polyester planar orientation.

A further embodiment of the invention calls for deposition of increasedthickness of metal at the edges of the film which are to be providedwith terminal contacts, in order to improve the strength of the contactand reduce t .e electrical resistance of the joint. This is particularlydesirable when the joint is actually transformed into a fused mass ofmetal particles and disoriented resin. The ratio of metal film thicknessat this edge to the metal film thickness in the body of the capacitor isat least 2:1 and preferably 3:1.

While the results are not'directly comparable to those achieved bypractice of the above embodiments, it is also possible to produce metalfilms on disoriented films of the polyesters set forth. Suchdisorientation may be accomplished by heating the polyester to atemperature at which the elastic memory that is, the relaxationtemperature, of the resin is permitted to function, but beneath thetemperature at which the resin melts. The metallizing on such films maybe carried on in any of the conventional manners known to the art.

The metallizecl films formed in accordance with this invention are ofparticular utility in condenser constructions. They may be wound intocondenser units possessing superior properties by virtue of thedielectrics employed. The coated polyester films set forth aresuccessfully used with other additional dielectric films such as filmsof polystyrene, impregnated paper, and the like. Such additional filmsmay be bonded directly to the polyester, or may comprise a separatesheet.

As many different embodiments of this invention may be made withoutdeparting from the spirit and scope hereof, it is to be understood thatthe invention is not limited to the specific embodiments hereof exceptas defined in the appended claims,

We claim: I 1. The process of bonding a metal film to a substantiallylinear thermoplastic planar oriented dielectric polyester sheetconforming to the recurrent formula:

0 0 mutations wherein R represents a bivalent radical containing thebenzene nucleus and R represents a bivalent aliphatic radical containingfrom 2 to 8 carbon atoms, which comprises condensing a metal'vapor onthe surface of said sheet and heating said surface to a temperatureabove its relaxation temperature, thereby disorienting said surfacewhich results in a firm and uniform bonding of the condensed metalthereto.

2. A process as defined in claim 1 in which said polyester is a linearcondensation polymer of terephthalic acid and ethylene glycol.

3. A process as defined in claim 1 in which said metal film consistsessentially of zinc superimposed on a layer of silver.

4. A process of firmly attaching a metal film to a substantially linearthermoplastic molecularly oriented sheet conforming to the recurrentformula:

wherein R represents a bivalent radical containing the benzene nucleus,and R represents a bivalent aliphatic radical containing from 2 to 8carbon atoms, which 0011]- prises condensing the vapors of a metaldirectly upon an unheated surface of said sheet, and heating thecombined metal film and polyester sheet at a temperature suificient torelax the planar orientation of said surface thereby firmly anduniformly bonding the condensed metal to the surface.

5. A metal coated resin film comprising a film conforming to therecurrent formula:

i ii R-CO- wherein R stands for a bivalent radical containing thebenzene nucleus, and R representsa bivalent aliphatic radical containingfrom 2 to 8 carbon atoms, having a planar oriented body and a thermallypartially disoriented surface, and a vapor-deposited metal filmuniformly bonded to said disoriented surface.

6. A coated film as defined in claim 5 in which said polyester is acondensation product of terephthalic acid and ethylene glycol.

7. A rnetallized polyester film comprising a partially disorientedpolyester sheet conforming to the recurrent formula:

wherein R represents a bivalent radical containing the benzene nucleus,and R represents a bivalent aliphatic radical containing from 2 to 8carbon atoms, and a vapor-deposited metal film uniformly bonded to adisoriented surface of said sheet, said film being obtained bycondensing a metal vapor on a surface of a planar oriented polyestersheet followed by heating or said surface until disoriented.

8. A film as defined in claim 7 in which said polyester isaco'ndensation polymer of terephthalic acid and ethylene glycol.

9. A. metallized polyester film comprising a partially disorientedpolyester sheet conforming to the recurrent formula:

wherein R represents a bivalent radical containing the benzene nucleus,and R represents a bivalent aliphatic radical containing from 2 to 8carbon atoms, and a vapor-deposited metal film uniformly bonded to adisoriented surface of said sheet, said metal film having been condensedon a planar oriented surface of the polyester film concurrent with thedisorientation of said surface by heating.

References Cited in the file of this patent UNITED STATES PATENTS KolbDec. 28, 1937 Kirby Mar. 2, 1943 Muskat et al. Oct. 19, 1943 McManus eta1. Aug. 14, 1945 Stoll Sept. 11, 1945 Swallow et a1. Feb. 14, 1950

1. THE PROCESS OF BONDING A METAL FILM TO A SUBSTANTIALLY LINEARTHERMOPLASTIC PLANAR ORIENTED DIELECTRIC POLYESTER SHEET CONFORMING TOTHE RECURRENT FORMULA: